In the petroleum refining industry, the fluidized catalytic cracking of hydrocarbons is well known and may be accomplished in a variety of processes which employ fluidized solid techniques. Normally in such processes, suitably preheated, relatively high molecular weight hydrocarbon liquids and/or vapors are contacted with hot, finely-divided, solid catalyst particles either in a fluidized bed reaction zone or in an elongated riser reaction zone, and maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons typical of those present in motor gasolines and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles in the reaction zone thereby reducing the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock. In order to restore a portion, preferably a major portion, of the activity to the coke-contaminated or spent catalyst, the catalyst is transferred from the reaction zone into a regeneration zone. Typical regeneration zones comprise large vertical cylindrical vessels wherein the spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas, such as air, under conditions to burn at least a portion, preferably a major portion, of the coke from the catalyst. The regenerated catalyst is subsequently withdrawn from the regeneration zone and reintroduced into the reaction zone for reaction with additional hydrocarbon feed.
In a fluid catalytic cracking unit (FCCU), commercial practice has been to employ fixed throat feed injectors. For example, Dean et al., U.S. Pat. No. 4,434,049 and Skraba, U.S. Pat. No. 4,575,414 disclose fixed throat injectors comprising atomizing spray nozzles. Chesmore et al., Japanese Kokai 59-145287 disclose a fixed throat feed injector with spiral momentum. Such fixed throat feed injectors are usually designed on a forecast basis and optimized for a certain feed quality. In the actual plant operation, however, feed quality is usually different from the forecast basis, since business objectives change with time. For these reasons, most conventional FCCU's change their fixed throat feed injectors on a two to three year cycle, which roughly corresponds to the FCCU turnaround cycle.
Furthermore, it is current practice with FCCU operations to practice multivariable constraint control to maximize refinery profits on a continuous basis. An important process variable is the product yield and quality. For example, the naphtha from a catalytic cracker is a large part of the mogas yield from a refinery. In fact, an FCCU is probably the single most important generator of valuable products in a refinery. Due to an FCCU's large throughput, even minor variations of yields can have a significant impact on economics.
Prior art methods of increasing product yield include changing the catalyst used and changing the physical reactor, for example shortening the riser of an FCCU, to achieve a shorter residence time.
There is a need for better and more continuous maximization of catalyst cracking fluid and overall refinery operation in a changing economic environment. During the course of a typical two-to-three year plant run, there is considerable room for increasing yields by continuous optimization.